[104697-92-9] · C26H33PSi · Tris(o-tolyl)[(2-(trimethylsilyl)ethylidene]phosphorane · (MW 404.65)
Preparative Methods: prepared in situ from tris(o-tolyl)[2-(trimethylsilyl)ethyl]phosphonium iodide and n-Butyllithium in THF.1 Tris(o-tolyl)phosphorus methylide (prepared in situ from methyltris(o-tolyl)phosphonium bromide and n-BuLi in THF) and (Iodomethyl)trimethylsilane were heated under reflux in THF for 5 h. The mixture was poured into H2O and the phosphonium salt was extracted with CH2Cl2. Evaporation of the organic solvent gave an oily residue which was crystallized from THF-Et2O (mp 111-113 °C). To a stirred suspension of the phosphonium salt in THF (6 mL g-1) was added 1 equiv of n-BuLi under a nitrogen atmosphere at 0 °C and the mixture was stirred for 30 min at rt, resulting in a clear yellow ylide solution.
Handling, Storage, and Precautions: unstable to air and moisture. Should be handled under a dry nitrogen atmosphere.
Phosphorus ylides having silyl groups at the b-position react with aldehydes by two pathways: a Wittig reaction and a vinylation reaction that affords allylic alcohols via migration of the silyl group and elimination of phosphine. [2-(Trimethylsilyl)ethylidene]triphenylphosphorane has been used for synthesis of allylsilanes from aldehydes.3 The reaction, however, shows low (Z/E) selectivity. In addition, allylic alcohol is formed as byproduct.3b,4 With modification of the substituents on the phosphorus atom from phenyl to the bulkier o-tolyl group, the title reagent reacts with aliphatic saturated aldehydes to give allylsilanes with high (Z) selectivity and without formation of allylic alcohols (eq 1).1
In the dichotomous reactivity of b-silylphosphorus ylides, electron-donating groups on the phosphorus atom and electron-withdrawing groups on silicon effect the selective formation of allylic alcohols over allylsilanes.2 Tris(4-methoxyphenyl)[(2-(methyldiphenylsilyl)ethylidene]phosphorane [110598-64-6] reacts diastereoselectively with chiral aldehydes to afford allylic alcohols with suppression of the Wittig reaction. With 2-phenylpropanal the erythro (syn) product predominates2 and with a-benzyloxy aldehydes (eq 2)5 and a,b-epoxy aldehydes6 the ylide gives erythro (anti) product selectively via the Felkin-Anh model. With a-N-Boc-amino aldehydes, [2-(trimethylsilyl)ethylidene]triphenylphosphorane gives the threo (syn) product preferentially through Cram's chelation transition state.7
Diastereoselective isopropenylation2 and (E) propenylation (eq 3)8 are also possible using b-silylphosphorus ylides substituted with methyl groups at the a- and b-positions, respectively. The latter ylide was prepared in situ by addition of methyldiphenylsilyllithium to the propenylphosphonium salt.8
Enantioselective vinylation of aldehydes (up to 92% ee) was reported by using a b-silylphosphorus ylide carrying a chiral ferrocenyl group on the phosphorus atom.9
Osaka City University, Japan